Our present invention relates to a method of and to an apparatus for wavelength detection in the determination of temperature by means of a fiber Bragg grating impressed in or written in a glass fiber. More particularly, the invention relates to a method of measuring a temperature in an electrical apparatus, particularly an electric transformer, utilizing a form of wavelength detection.
A Bragg grating impressed in or written into a glass fiber has, by virtue of its characteristic geometry, a wavelength selective property. Only a respective reflected Bragg wavelength having a limited bandwidth, is reflected by such a grating when a broad-band light is conducted thereto. This property of reflecting a limited bandwidth light is dependent upon external influences like temperature or distortion of the Bragg and enables a fiber Bragg grating to be used as a sensor, especially for temperature. The evaluation of the signal in the past has been limited to a highly precise wavelength detection in the picometer to nanometer range. The Bragg reflection wavelength is directly dependent upon the glass fiber temperature and thus the temperature in the vicinity of the glass fiber at the measurement location which can be in an electric transformer.
U.S. Pat. No. 5,513,913 to Ball et al discloses the provision of a plurality of fiber Bragg gratings in a glass fiber which act independently of one another. It is possible to identify the different Bragg gratings and thus, based on the slight shift in the Bragg reflection wavelength with temperature variation at each measurement location, utilizing a broad-band light source, to determine a number of different temperatures at a number of different locations. Each of the Bragg gratings written into the glass fiber can then be disposed at the respective measurement location.
U.S. Pat. No. 5,493,113 to Dunphy et al describes a similar arrangement in which the glass fiber in which the Bragg grating is inscribed is partly fixed in a tube, i.e. is surrounded by a capillary. The evaluation of the signal obtained has been not described in detail in this patent which only mentions detectors.
However, all of the known arrangements of the aforedescribed type determine temperature changes at the measurement location by variations of the reflection wavelength and thus evaluate the latter to obtain information as to the measured temperature. The linear dependency of the reflection wavelength upon the glass temperature can be detected and converted into a temperature measurement with the aid of commercial optical spectrum analysis, for example, the Hewlett Packard type HP 71450B, 7125B, 71452B and 86140A or the series WA spectrum analyzers of Burleigh.
A drawback in the use of such analyzers for fiber Bragg temperature measurements utilizing wavelength detection and spectrum analyzers is the high cost of the measurement apparatus and the fact that such apparatus is overdimensioned for the type of wavelength detection which is required.
German patent document DE-A 198 21 616 describes an apparatus for determining the temperature and strain of an optical fiber utilizing a broad-band light source, a fiber coupler to couple the light from the light source into the optical fiber which can be inscribed with one or more Bragg gratings and for coupling the reflected light from the Bragg grating into a further glass fiber. An evaluating unit is coupled to the second glass fiber to evaluate the optical signal delivered thereby. The evaluating unit, to which the second glass fiber is connected, is in turn an optical input, two optical splitters for transmitting the light along two distinct optical paths, means for generating two interferometer paths as different optical wavelengths and a phase modulator in one of these paths. The interferometer arrangement supplies two optical outputs which are subjected to spectral analysis. The apparatus therefore also requires special spectral analyzing means which can be expensive and may be excessive for the kind of wavelength detection required for temperature measurements utilizing a Bragg grating.
The principal object of the present invention is to provide a simple and economical process or method for detecting wavelength utilizing a Bragg grating system or, more specifically, for measuring a temperature, especially in an electrical apparatus, whereby drawbacks of earlier systems re avoided.
Another object of the invention is to provide a low-cost wavelength detection system for use in the measurement of a temperature in an electrical apparatus, especially a transformer, which in spite of its simplicity, enables an exact measurement of wavelength change with temperature variation and which thus can be utilized for highly precise temperature measurements in such apparatus.
It is also an object of the invention to provide an improved apparatus for carrying out the method.
These objects and others which will become apparent hereinafter are attained, in accordance with the invention in a method of wavelength detection for the measurement of temperature by means of a glass fiber whereby:
into a first glass fiber with a Bragg grating with a specific wavelength xcexBG1, broad-band light is launched,
the first glass fiber is optically coupled with a second glass fiber by an optocoupler,
the first glass fiber with its first Bragg grating is introduced into an electrical apparatus, usually a transformer, to position the first Bragg grating at a location at which a temperature is to be measured,
a temperature increase in this region shifts the specific wavelength xcexBG1 of the first Bragg grating,
the temperature-dependent reflected wavelength portion of the first Bragg grating is supplied to the second glass fiber, i.e. coupled thereto, and the light coupled into the second glass fiber is then evaluated with an output signal of the evaluating unit being a measure of the temperature of the region of the electrical apparatus whose temperature is to be determined.
According to the invention, in the second glass fiber, a second Bragg grating II with a specific reflection wavelength xcexBG2 is provided whereby xcexBG2 is different from xcexBG1, the light reaching the second Bragg grating is that which was reflected from the first Bragg grating, and the nonreflected portion of the light traversing the second Bragg grating is fed to a photodetector whose output voltage is thus dependent on light intensity (i.e. is reduced as a function of light intensity), and thus a measurement of the temperature.
The method of measuring the temperature in the electrical apparatus thus comprises the steps of:
(a) launching a broad-band light into a first glass fiber impressed with a first Bragg grating having a specific Bragg reflection wavelength xcexBG1;
(b) optically coupling the first glass fiber with a second glass fiber impressed with a second Bragg grating having a specific Bragg reflection wavelength xcexBG2 different from the specific Bragg reflection wavelength xcexBG1 of the first Bragg grating and so coupled with the first glass fiber that reflected light from the first Bragg grating is conducted to the second Bragg grating;
(c) introducing the glass fiber into an electrical apparatus to position the first Bragg grating at a location at which a temperature is to be determined, whereby the Bragg reflection wavelength xcexBG1 of the first Bragg grating is shifted as a function of change in the temperature at the location; and
(d) feeding nonreflected light from the second Bragg grating to a photodetector having an output voltage dependent upon detected light intensity and representing a measurement of the temperature at the location.
Preferably the first glass fiber is formed with a plurality of the first Bragg gratings, the method further comprising positioning each of the first Bragg gratings at different locations in the electrical apparatus at which respective temperatures are to be measured, and varying the specific Bragg reflection wavelength xcexBG2 of the second Bragg grating by mechanically deforming the second glass fiber in a micrometer range.
The specific wavelengths of all of the glass fibers are so dimensioned that, upon a measured temperature in the electrical apparatus exceeding a predetermined critical temperature, an output voltage level at the photodetector will exceed a limiting value and automatically generate an alarm signal.
The apparatus for measuring the temperature in the electrical apparatus can comprise:
a first glass fiber impressed with a first Bragg grating having a specific first Bragg reflection wavelength xcexBG1 and positioned at a location in an electrical apparatus at which a temperature is to be measured, whereby the Bragg reflection wavelength xcexBG1 of the first Bragg grating is shifted as a function of change in the temperature at the location;
a source of broad-band light coupled to the first glass fiber for launching the broad-band light into the first glass fiber;
a second glass fiber impressed with a second Bragg grating having a specific second Bragg reflection wavelength xcexBG2 different from the specific Bragg reflection wavelength xcexBG1 of the first Bragg grating;
an optocoupler for coupling the first glass fiber with the second glass fiber so that reflected light from the first Bragg grating is conducted to the second Bragg grating; and
a photodetector coupled to the second glass fiber downstream of the second Bragg grating and receiving nonreflected light from the second Bragg grating, the photodetector having an output voltage dependent upon detected light intensity and representing a measurement of the temperature at the location.
Preferably a plurality of spaced-apart first Bragg gratings are written into the first glass fiber and are positioned at a corresponding number of locations of the electrical apparatus at which temperatures are to be measured, and the second Bragg grating has a variable second Bragg reflection wavelength xcexBG2.
The photodetector can comprise a photodiode followed by a transimpedance amplifier.
The apparatus can also comprise means for mechanically deforming the second glass fiber in a micrometer range to vary the specific second Bragg reflection wavelength xcexBG2 of the second glass fiber.
The optocoupler can have a branch to which a further glass fiber is coupled, the apparatus further comprising means for converting a light signal in the further glass fiber to a voltage, an output signal of the photodetector being normalized to the voltage into which the light signal in the further glass fiber is converted.
The invention is based upon the fact that the detection of the environmentally affected and here temperature-dependent Bragg wavelength can utilize the steep characteristic of the second Bragg grating with a slightly shifted central wavelength.